The present invention relates to a brake-pressure control device for a road vehicle having a hydraulic two-circuit brake system in which, the device is equipped with an anti-lock brake system (ABS) which operates according to the recirculation principle and with a device for dynamic movement control (FDR).
A brake-pressure control device is the subject of a non-prepublished patent application P 43 29 139.2 and was conceived for a road vehicle with front axle/rear axle brake circuit distribution. A static main cylinder constructed as a tandem main cylinder was provided as a brake-pressure control device with pressure outputs individually assigned to the brake circuits. The dynamic movement control device operated according to the principle of keeping the longitudinal slip and the lateral slip of the vehicle wheels within limits which are compatible overall with dynamic stability of the vehicle by electronically controlled, automatic activation of one or more wheel brakes.
The recirculation pump or pumps of the known anti-lock brake system is or are utilized as pressure sources, from which the wheel brakes utilized for the control can be supplied with brake pressure in dynamic movement control mode and can be supplied on the input side with the output pressure of a precharging pump which operates at a relatively low output pressure level of, for example, 15 bar and is electrically driven.
In order to separate the two brake circuits hydraulically, each circuit is provided with individually assigned precharging cylinders which have one piston each which separates an output pressure space in a pressure-tight way from a drive pressure space to which the pressure output of the precharging pump is connected. The output pressure space can be connected, via an electrically actuable precharging control valve, to the low pressure port of the recirculation pump of the respective brake circuit. Two flanges are arranged at an axial distance from one another and between which extends an overrun space which is kept in constant communication with, in each case, one of the pressure outlets of the brake device.
The piston of the respective precharging cylinder is provided with a valve which can be activated mechanically and also fulfills the function of a non-return valve which blocks a through-flow channel of the piston which connects the overrun space to the output pressure space, for as long as the pressure in the output pressure space of the respective precharging cylinder is greater than in the overrun space and clears this through-flow path when the main cylinder output pressure applied to the overrun space is greater than the pressure in the output pressure space of the precharging cylinder.
Between the pressure output of the respective precharging cylinder and the main brake line of the brake circuit which is supplied with pressure, in each case an electrically actuable change-over valve is connected to be open when deactivated and to block in the excited state which is assigned to dynamic movement control mode. The low-pressure inputs of the two recirculation pumps are connected to the main brake line of the respective brake circuit via in each case one non-return valve which is acted on in the opening direction by relatively higher pressure in the return line of the respective brake circuit than at the low-pressure inlet of its recirculation pump, and is otherwise blocked. The two precharging cylinders have a common drive pressure space which is connected to the pressure output of the precharging pump and can be blocked off from the brake-fluid reservoir container of the brake system by actuating a solenoid valve which is open when deactivated.
The considerable technical outlay and spatial requirements associated with the two precharging cylinders and increased by the need for a precharging pump, must be seen as disadvantages of the brake-pressure control device according to the above-mentioned patent application P 43 29 139.2. The brake-pressure control device is of substantially more complicated configuration, and also correspondingly more expensive, compared to a vehicle which is equipped with an anti-lock brake system and a traction control system which permits only the wheel brakes of the driven vehicle wheels to be activated automatically. In addition, in a brake system equipped with the brake-pressure control device explained here, additional chambers are also present because of the additional precharging cylinders, and gas bubbles which can form when the wheel brakes are subjected to high thermal stresses can only escape with difficulty from the chambers. This is particularly so if the two precharging cylinders are combined to form a "symmetrical" constructional unit. Therefore, the arrangement of the two precharging cylinders which is most favorable for them to be vented at all satisfactorily can, at most, be the best possible compromise.
An object of the present invention is, therefore, to improve a brake-pressure control device to such an extent that it is possible to achieve increased functional reliability of the control device with an apparatus which is still simpler and more cost effective.
This object has been achieved according to the present invention by a brake pressure control device in which the recirculation pump of the rear-axle brake circuit is configured as a self-priming pump. Moreover, the precharging cylinder is configured to be activatable in order to increase pressure in the output pressure space by one of (i) valve-controlled application to a drive pressure space of the precharging cylinder of output pressure of a built-in vehicle hydraulic pressure source vehicle, wherein the output pressure from one of a hydraulic power steering system and a permanently driven hydraulic ride leveller pump and (ii) valve-controlled venting of a drive chamber constituting the drive pressure space previously connected to a built-in vacuum source of a pneumatic driving cylinder and output pressure of a built-in, pneumatic pressure source to the pneumatic cylinder.
Accordingly, instead of a non-self priming recirculation pump, usually an eccentric-operated free-piston pump, a self-priming recirculation pump is provided for the rear-axle brake circuit. The self-priming recirculation pump can be a piston pump whose piston is continuously forced by a restoring spring into a position which corresponds to the maximum volume of the pump chamber so that the pump chamber into which brake fluid can continuously flow on, via an input non-return valve which has an opening pressure which is significantly lower than 1 bar, e.g. only 0.2 bar, is reduced in volume when the piston is displaced by the compression stroke and brake fluid can be forced, via the output non-return valve of the pump, into the connected main brake line of the rear-axle brake circuit II.
By virtue of this recirculation pump, automatically controlled braking phases can be ensured at the rear wheel brakes solely by actuating the recirculation pump. The rear-axle brake circuit is shut off from the rear-axle pressure output of the brake device by its change-over valve, but the rear-axle pressure output in return is connected to the low-pressure input of the recirculation pump via a supply control valve.
A precharging cylinder is provided for the front-axle brake circuit which is hydraulically separated from the rear-axle brake circuit II. The precharging cylinder can be activated in order to increase pressure in its output pressure space in the event of an automatically controlled activation of one or more front-wheel brakes in order to feed brake fluid into the pump chamber of an associated recirculation, non-self priming pump by valve-controlled application to a drive pressure space of the precharging cylinder with the output pressure of a built-in hydraulic pressure source of the vehicle, e.g. the output pressure for supplying pressure to a hydraulic power steering system or the output pressure of a permanently driven pump of a hydraulic ride leveller. Alternatively, this can also take place by valve-controlled venting of a drive chamber already connected to a built-in vacuum source, e.g. the vehicle engine suction connector, of a pneumatic cylinder or by applying to the same the output pressure of a built-in pneumatic pressure source such as the pressure supply unit of a pneumatic suspension.
As a result of supplying drive pressure to the precharging cylinder which is provided for supplying brake fluid to the recirculation pump of the front-axle brake circuit I by valve-controlled connection of its drive cylinder, which can be of simple configuration, to a pressure source or vacuum source, which is already present in the vehicle, the precharging pump which is otherwise necessary is omitted and is, so to speak, replaced by a pressure source or vacuum source which is already present on the vehicle. Compared with the brake-pressure control device according to the aforementioned patent application p 43 29 139.2, the obstacle for realizing an otherwise functionally equivalent brake-pressure control device is reduced by a precharging cylinder which is assigned to the rear-axle brake circuit II and the precharging pump which is necessary in the aforesaid brake-pressure control device. The saving in terms of technical outlay and spatial requirements thereby achieved can be considered a considerable technical advantage. Moreover, the arrangement of the precharging cylinder for the front-axle brake circuit I can be achieved without compromise in order to obtain better venting.
Of course, by configuring the recirculation pump of the front-axle brake circuit I as a self-priming pump, according to the present invention for the rear-axle brake circuit II, an overall, even more far-reaching, simplification of the brake-pressure control device is achieved. That is, with a corresponding configuration of the recirculation pump of the front-axle brake circuit I, the precharging cylinder which is provided for the said brake circuit I can be eliminated and a built-in pressure source or vacuum source provides the operational capability. Such a "consistent" simplification of the brake-pressure control device to the largest possible degree in comparison with that described in the aforementioned patent application P 43 29 139.2 is, however, hindered by the fact that the functional reliability of such a self-priming recirculation pump is somewhat less than that of one which can be operated with, so to speak, forced feeding of the pressure medium so that it appears expedient to operate the pressure source for the front-wheel brakes, i.e. the recirculation pump of the front-axle brake circuit I, with precharging pressure in the automatically controlled braking mode. This applies in particular to a situation in which the ambient temperature is very low and therefore the viscosity of the brake fluid can be high.
The configuration which is preferably selected for the auxiliary cylinder according to the present invention and its drive cylinder has the advantage that there is an annular space which can be utilized as a media separation space, and can be expediently vented, i.e. is kept below the pressure of the ambient atmosphere, can, if appropriate, also be connected to the suction connector of the vehicle engine. The media separation space can be connected to the brake fluid reservoir container in the event that the drive cylinder of the auxiliary cylinder is constructed as a pneumatic cylinder.
As a result of the auxiliary cylinder/drive cylinder assembly according to the features of the present invention, reliable separation of media is achieved in the event that the drive cylinder is operated with a hydraulic oil. For example a mineral oil is customarily used and must not be mixed with brake fluid if damage to the piston seals of the auxiliary cylinder piston and/or of the drive piston is to be reliably avoided.
If the piston of the drive cylinder has a cross-section which is different from the piston of the auxiliary cylinder, these pistons are advantageously constructed as separate components in order to be able to compensate easily for production tolerances, such as small eccentricities in the bore hole stepping of a common cylinder housing.
If the supply pressure which is required for activating the precharging pressure source, for its drive cylinders, is derived, according to the features of the present invention, from the output pressure of a permanently driven hydraulic pump and can be adjusted according to the principles disclosed herein, the brake-pressure control device itself can be very easily adapted to a pressure supply unit which is present on the vehicle.